1. Field of the Invention
This invention relates to a method and an apparatus for producing homoepitaxial diamond thin film on a substrate surface by plasma assisted CVD using a mixed gas of a carbon source and hydrogen reaction gas.
2. Description of the Prior Art
The semiconductor IC, now used widely in low-voltage applications, is increasingly being seen as also having strong potential in high-voltage power applications. This marks the advent of the power electronics era. The main factor limiting the performance of a power electronics device is the heat resistance of its material. Si, the primary material used in semiconductor development up to now, is usable only at temperatures under 200.degree. C. On the other hand, diamond is a wide-band-gap semiconductor with excellent physical and chemical properties enabling operation even at 1000.degree. C.
The electron structure of a crystal usually prevents electrons from assuming energies in what is referred to as the energy band gap. Unless excited by an energy such as heat or light, electrons below the band gap (in the valence band) cannot move to a high-energy state above the band gap (to the conduction band). If a substance constituting an electron source at room temperature (an n-type substance called a donor) is introduced into the semiconductor near the upper end of the band gap, conduction electrons (serving as information media in an electronic device) are supplied into conduction band. If a substance that absorbs valence electrons (a p-type substance called an acceptor) is introduced into the band gap near the valence band, holes serving as information media are supplied into the valence band. When the environment is such that electrons cross the band gap from the valence band to the conduction band to form electron-hole pairs, the electronic device cannot function (intrinsic region). The environment robustness of an electronic device is determined solely by band-gap width. The band gap of Si, the mainstay of today's semiconductor industry, is 1.11 eV. That of diamond is 5.45 eV. Therefore, Si loses its n-type or p-type property at around 300.degree. C. but diamond can retain its n-type or p-type property even at temperatures above 1000.degree. C. Owing to its large band gap, diamond also resists electron-hole formation by high-energy radiation. In addition, diamond's high hole mobility and small dielectric constant enable high-frequency device operation, while its high heat conductivity facilitates device miniaturization and high-density integration. Owing to its chemical stability, high heat conductivity, negative or slight electron affinity, and other properties not possessed by other electronic materials, diamond has also recently come to be seen as a promising material for the high-efficiency electron emitters used in flat panel displays. Diamond can thus be considered an ideal material for both conventional electronics applications and in all sectors of power electronics including those related to next-generation energies.
Such use of diamond thin film as a semiconductor material requires the capability to produce semiconductor-grade homoepitaxial diamond film with atomic level surface smoothness (flatness) and substantially no defects or impurities.
High-grade homoepitaxial diamond film exhibiting these qualities is known to be producible by plasma assisted CVD with the concentration of the carbon source gas set to not higher than 0.05%.
This method is not practical, however, because the film forming rate depends on the carbon source gas concentration and is 0.01 .mu.m/h when the concentration is 0.05% or lower.
When the carbon source gas concentration is increased in an attempt to speed up film formation, the thin film formed comes to include many impurities and defects and cannot be used as a semiconductor material.
An object of the invention is to provide a method and apparatus for producing high-quality homoepitaxial diamond thin film usable as semiconductor material at a practical film forming rate.